Well, for starters there is the "johnny cochran" issue: not legal to modify an appliance like that. You violate it's GAMA/AGA/UL listing, and hence lose the permit it was installed on.
Aside from that, now you don't have the air flow balanced against the heat produced. That can't work out well. \

And most important, you affect the operation of safety circuits like hi temp limit switches, etc. Those were all designed around a certain burner/fan operating scenario, and now...who knows.

If you don't need 100,000 BTU, a whole new furnace is not that expensive, and if you want uber energy efficiency, you get a new furnace which is DESIGNED to operate the burners in stages, and modulate the air flow accordingly. The unit costs more, but in a very cold climate can save a lot of energy. If you are upstate like in Buffalo or something, this would be worth doing.

Most gas burners (at least should be) adjusted for proper air/fuel mix to minimize carbon and CO production. Just changing the gas (fuel) would disrupt that balance, and in itself would be unsafe unless you rebalanced the air/fuel mix. As already mentioned, just arbritraily going in and changing things is likely to make for an inefficient system and not increase your comfort or safety. The heat exchange output temp, drafting in the flue, blower speed, and other things would likely be compromised. The reduced heat output might also lead to condensation and self-destruction of the interior since it would not get as hot. Really not a good idea unless it was designed for it (some are, within limited range).

I've heard there are reasons not to reduce the output of a furnace by eliminating burners (going from a 100btu down to a 35btu). What are the reasons not to do this?

Thanks,
Bill

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The massive excess combustion air will reduce the steady state combustion efficiency, but you'd also suffer flue condensation issues unless you narrowed it down with a liner. At that large a reduction you'd end up with laminar flow on fire-side of the heat exchangers (unless it has mechanically drafted venting), reducing the heat transfer efficiency.

Derating a low-mass hot air-furnace has negligible effect on efficiency, even when done in a range that doesn't cause other problems. Derating a high-mass boiler can sometimes lead to fuel savings, even it's steady-state efficiency is slightly reduced, but even then the "comfort & efficiency" money is usually better spent elsewhere.

I hope you added some Lube with that Ethanol, Sounds like it ran fast for a short period of time. lol

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Actually, it ran pretty well once it warmed up, which took a while, even in nice weather. I did not run it long enough to damage any valves. Used about 15 gallons of Ethanol in total during my experiments.

Actually, it ran pretty well once it warmed up, which took a while, even in nice weather. I did not run it long enough to damage any valves. Used about 15 gallons of Ethanol in total during my experiments.

Serious business. A "furnace" should never be downsized, as the tin heat exchanger will not last long voiding warranty etc. A cast iron "boiler" could be de-rated, but the added stress to cold sections makes them fail prematurely, (so say the manufacturers).

As pointed out early in this thread, all gas-fired appliances are UL/CSA tested and approved in a certain configuration and output. Changing orifices and plugging burners is not good practice and should never be attempted without the aid of an experienced HVAC technician with a current combustion analyser.